The present invention relates to compounds which are suitable for use in magnetoelectronics. Such compounds, owing to their magnetoresistive properties, can be used inter alia as sensors for magnetic fields. Magnetoresistive properties are intended to mean a change of resistance under the effect of an external magnetic field (magnetoresistance effect).
Magnetic sensors are employed in the magnetic heads of hard disk drives, which are used for example as external storage devices for computers. Conventional magnetic heads detect the strength and direction of a magnetic field on the basis of a current which is induced in coil. When the recording density increases, however, the recording area for a bit decreases and the resulting magnetic fields become progressively weaker. In order to detect such small external magnetic fields, highly sensitive magnetic sensors are required. Magnetic sensors which use a magnetoresistance effect (giant magnetoresistance=GMR effect or tunneling magnetoresistance=TMR effect) are known to be such magnetic sensors with high sensitivity (S. Mengel, Innovationspotential Magnetoelektronik [Innovation Potential of Magnetoelectronics], Physikalische Blatter, 55(5) 1999, p. 53-55).
Magnetoresistance is a quantity which describes the percentage change of the resistance of a system with and without an external magnetic field. Magnetoresistance is defined as the decrease or increase of the electrical resistance under an external magnetic field, relative to the resistance without a magnetic field. In general, two different definitions are used for the percentage magnetoresistance. The definition used here for the percentage magnetoresistance MR0 is given by the difference between the resistance without a field and the resistance with a field, divided by the resistance without a field (in %). The maximum value is 100%. An alternative definition (for example DE-A 38 20 475, whose United States equivalent is U.S. Pat. No. 4,949,039) is used in the case of the so-called GMR (giant magnetoresistance) effect. This effect is found in multi layer systems. In the simplest case, these layer systems consist of two magnetic layers, for example iron, which are separated by a nonmagnetic interlayer, for example chromium. In this case, the percentage magnetoresistance MRp is given by the difference between the resistance without a field (high resistance, owing to the antiparallel spin orientation of the two magnetic iron layers) and the resistance with a field (low resistance owing to the parallel spin orientation of the two magnetic iron layers) divided by the resistance with a field (parallel spin orientation of the two magnetic iron layers) in %. The maximum value can be arbitrarily large. The GMR effect, which has been known since 1987, is likely to be an interfacial effect (DE-A 38 20 475). Whenever the two iron layers are coupled ferromagnetically via the chromium layer, the resistance is low since the electrons can pass into the second iron layer without changing their spin. In the case of antiferromagnetic coupling between the spins of the two iron layers, on the other hand, the resistance is high.
Ferromagnetism is generally defined as a collective magnetism in which the electron spins are aligned parallel below the critical temperature (Curie temperature). Antiferromagnetism is defined as collective magnetism in which the electron spins are aligned antiparallel below the critical temperature (Néel temperature).
The resistance of a layer system with antiparallel coupling between the iron layers can be reduced significantly by an externally applied magnetic field. The external magnetic field forces ferromagnetic alignment of the spins of the two iron layers in the direction of the field. At most a 10% MR0 effect can be achieved at room temperature by this effect. The GMR effect has already entered technical use, in particular as a magnetic sensor in read heads for hard disk drives (IBM 1997).
The importance of these magnetoresistance materials for magnetic sensors and magnetic data storage has grown immensely in recent years and is already of great commercial importance. Electronics based on electron spin is also on the verge of entering the market. The magnetic materials used in this case are almost exclusively 3d metals (that is to say metals with partially filled 3d orbitals), since these metals exhibit the desired magnetic properties and are substantially compatible with the process techniques in silicon technology. Their principal characteristics, for example the size of the magnetoresistive effect, maximum working temperature (operating temperature of the magnetic sensors), field sensitivity etc. have been improved in recent years, but without an optimal solution having been found for all areas of application.
In 1993, large magnetoresistances were also discovered in compounds (in contrast to layer systems, see above). The “colossal magnetoresistance” CMR effect in manganese oxides has attracted great interest worldwide (R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) page 2331) since the resistance change when applying an external magnetic field is much more than in layer systems. This CMR effect is an intrinsic effect, and the extremely large resistance change is due to the suppression of a metal-insulator transition at the Curie temperature TC.
The Curie temperature is defined as the critical temperature below which spontaneous magnetization occurs with a parallel arrangement of the spin moments on neighboring atoms, also referred to as a ferromagnetic arrangement. Above TC, the spins are disordered and the compounds which exhibit a CMR effect are insulators (semiconductors), while below TC these compounds are ferromagnetic metals. The effect is therefore usually greatest at the Curie temperature. A technological application is also sought in this case, although the greatest effect is usually found at temperatures below room temperature (not in the operating temperature range of read heads).
In 2000, the so-called EMR effect (extraordinary magnetoresistance) was discovered. It is a geometrical effect, in which the magnetic field prevents the flow of electrons between a semiconductor and a metal. This effect was shown for the first time on indium-antinomy van der Pauw disks which have gold embedded in the middle (S. A. Solin, T. Thio, D. R. Hines, J. J. Heremans, Science 289 (2000) 1530). With a zero field, the current flows primarily through the gold (the metallic “inhomogeneity”), but when there is an applied magnetic field, the electrons can no longer pass from the indium-antimony semiconductor into the gold. This leads to high magnetoresistances, even though the materials themselves show no physical magnetoresistance. In contrast to the GMR and CMR effects, the EMR effect is always positive, i.e. the resistance increases in the magnetic field. The application potential of EMR components has already been studied (J. Moussa, L. R. Ram-Mohan, J. Sullivan, T. Zhou, D. R. Hines, S. A. Solin, Phys. Rev. B 64 (2001), 184410), but it is not yet important in contemporary magnetoelectronics.
A feature common to all the magnetic field sensors mentioned so far is that they have to be produced elaborately in a plurality of process steps. TMR and GMR sensors consist of a system of thin layers of different materials. These thin layers are produced for example by sputtering, laser ablation or molecular beam epitaxy (MBE), or are vapor deposited (see for example S. Falk, dissertation, Mainz, 2004). EMR sensors consist of a metallic material embedded in a semiconductor material (S. A. Solin, T. Thio, D. R. Hines, J. J. Heremans, Science (2000) 289, 1530).
Recently, inhomogeneous semiconductor compounds of the half-Heusler type have been reported (Los Alamos National Laboratory, Preprint Archive, Condensed Matter (2007), 1-3, arXiv:0709.4182v1) which likewise exhibit a GMR and EMR effect. These inhomogeneous compounds are produced by adding, to the stoichiometric composition, an excess of a metal which the compound comprises and which is distributed randomly as “islands” in the matrix of the compound; complicated encapsulation or embedding which is elaborate in terms of processing technology, as in the In—Sb with embedded Au, is obviated here. However, the described inhomogeneous half-Heusler compounds have the disadvantage that the resistance effect on the one hand occurs only at very low temperatures (10-100 K) and on the other hand—which is more important—the percentage resistance change is negative.
Furthermore, nonstoichiometric compounds which have a high positive magnetoresistance are known—Ag2+δSe and Ag2+δTe (R. Xu, A. Husmann, T. F. Rosenbaum, M.-L. Saboungi, J. E. Enderby, P. B. Littlewood, Nature (1997) 390, 57). Here, however, production is much more difficult and involves working with highly toxic materials.